Various types of optical devices are well-known in the field of electron microscopy to create a magnified image of an object, feature or component that is generally too small to be seen by the naked eye. Such devices may include the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning Transmission Electron Microscope (STEM). Imaging of a sample is typically achieved by detecting emanations or output radiation from the sample caused by irradiation of the sample by the imaging beam. Such emanations may include secondary electrons, backscattered electrons, X-rays, light (visible light and near visible light such as near-IR and near-UV), and any combination of these. These imaging devices may be part of a “dual beam” system with an additional tool having a machining function to modify a sample by removing material, such as by milling or ablation, or adding material, such as by deposition. For example, such a dual beam system may include a Focused Ion Beam (FIB) for machining and an electron microscope for imaging.
In a typical dual beam system, each device or column is positioned as close to the sample as possible to enhance resolution and obtain good optics while providing enough space for various detectors or other various accessories to collect and detect emanations from the sample for imaging. However, space is typically limited near the point where the beams impact the sample. Because of space limitations each column is generally formed with a conical end allowing the column ends to abut so that they both can be located as near the sample as possible. In order to fit both columns into the space the ends of each column are necessarily spaced a working distance from the sample that typically may be between 13-16 mm. Such working distances have the advantage of providing room for various accessories, such as secondary electron detectors, backscattered electron detectors, and x-ray detectors. However, such working distances degrade the optics resulting in less than optimal imaging.
One example of such a dual beam system is shown and described in U.S. Pat. No. 6,373,070 to Rasmussen for “Method Apparatus For A Coaxial Optical Microscope With Focused Ion Beam.” A beam system 10 includes a focused ion beam column 12 and an optical microscope column 14 for observing a specimen 16. The relationship between the two columns 12 and 14 is best seen in FIG. 2A. The system 10 includes a lens tube 32 that supports a camera 40, an illumination system 42, and a lens assembly 44. Light emitting diodes (LEDs) 70a and 70b illuminate the specimen 16 and a mirror assembly 56 collects light from the specimen 16 and reflects it into the lens assembly 44 for observation and/or imaging. It can be seen that the end of each column 12 and 14 is formed with a conical shape so that the columns 12 and 14 can be positioned in an abutting relationship to fit inside the small space above the specimen 16. This arrangement provides for adequate detection of emanations from the specimen 16 because it allows for space for positioning of each end of the columns 12 and 14 plus additional accessories such as the mirror assembly 56 and diodes 70a and 70b. However, in order to provide such space the columns 12 and 14 are necessarily spaced from the specimen 16 a relatively large working distance that degrades the optical quality of the image of the specimen 16.
Improved optical quality can be obtained with devices capable of working with smaller spot sizes, which require a smaller working distance between the sample and the optical column. This can be achieved by using a standard optical column alone but is difficult when it is desired to use a dual or multiple beam system. One way to achieve smaller working distances in a multicolumn system is to us smaller optical columns. Some columns are manufactured to be smaller than typical columns and are constructed to be as small as possible. Smaller or “mini” columns offer the advantages of being more economical to manufacture and requiring less space. Mini-columns can be used alone or as part of a multi-column system because there is no requirement for coincident beams the columns do not have to be crowded together in a small space. Therefore, the ends of each column can be located much closer to the sample than typical beam columns allowing for much smaller working distances enabling smaller spot sizes at the sample providing better optics. However, whether using a stand-alone column or a multi-column system a small working distance makes detecting of emanations from the sample more difficult due to the angle at which emanations from the sample are projected and lack of space for detectors.
A typical charged particle optical column employing an electrostatic final lens includes one or more electrodes in the final lens through which the primary beam is projected. The electrodes control and focus the primary beam onto the sample. When it is desired to form an image by detecting secondary electrons or backscattered electrons a through-the-lens (TTL) detection system can be used in which the electrodes are biased to draw the electrons from the sample up through the lens. The electrons are then detected by an element, such as a channel plate electron multiplier, located within the lens. However, current TTL detectors are not useful when it is desired to form an image using X-rays or reflected light from the sample because the electrodes within the lens would block the X-rays and light.
What is needed is a charged particle optical column that is capable of use with small working distances from the sample while allowing for collection of emissions, such as X-rays and/or light, for detection and imaging.